Method of making a prestressed concrete structural member



Oct. 1, 1968 G. J. YEVICK 3,403,607

METHOD OF MAKING A PRESTRESSED CONCRETE STRUCTURAL MEMBER Original Filed Oct. 16, 1961 "r 4; GEORGE J. YEVICK Q INVENTOR} United States ABSTRACT OF THE DISCLOSURE A method of making a prestressed concrete structural member. Individual concrete blocks are radially inwardly compressed over a continuous circumferential zone. The concrete blocks are formed in the cells of a reticulate member, the cells expanded as by heating, shims are placed in the clearance spaces, and the cells allowed to shrink.

This is a divisional application of my copending application, Ser. No. 147,703, filed Oct. 16, 1961, now Patent No. 3,228,309, entitled Prestressed Unit.

It is known in the structural arts that economies may be effected by the utilization of certain materials such as concrete which have been prestressed. In their normal state such materials often exhibit a greater resistance to compressive stresses than to tensile stresses. By various schemes and methods, structural units fashioned from such materials may be placed in a desired state of compression so that during their subjugation to external structural loads, the resultant stresses within the material may be withstood without failure of the material.

Prior methods of introducing desired stress into a structural unit formed of such a material have often utilized a tensioned, elongated rod of steel or other substance having a high tensile strength which passes through the unit. The resultant compressive force due to the tensioned member acts against two opposed points or sides of the unit where the ends of the tensioned member, often provided with bearing plates, contact it.

By the present invention, however, compressive force acts over a continuous circumferential Zone of the material thus giving rise to a plurality of oppositely directed compressive stresses. Further, the invention utilizes this novel compressive stress distribution in combination with a reticulate configuration of the steel or other substance having a high tensile strength within the structural unit. Thus, the advantages of the above novel stress distribution are combined with the excellent strength-to-weight ratio of a reticulate structural configuration.

According to the invention, a prestressed structural unit is formed of a reticulate member whose cells or interstices contain blocks which are compressed over continuous circumferential zones or portions of their peripheries. The unit is formed by cross-sectionally expanding the cells or interstices of the reticulate member and inserting the blocks therein. The cells are then made to circum-ferentially contract about the blocks. The structural unit may be used as a modular unit of construction.

One example of the utility of the invention is afforded by its use as a roadbed. In this example the reticulate member is fashioned from steel and the blocks are of concrete. As is well known, many concrete roadbeds fail due to tensile stress in the concrete caused by low temperatures and load changes. By prestressing each concrete block in accordance with this invention, it may be caused to remain under compressive stress during wide temperature and load variations thus precluding failures due to tensile stresses within the material.

A difiiculty encountered in the fabrication of the subject prestressed units is that of matching the cross-section of the cells or interstices of the reticulate member with that of the blocks which are placed therein. Even slight variations in the lengths of the sides of the cells in the reticulate element or the lengths of the sides of the blocks, as well as undulations in the walls of the cells or the walls of the blocks, will cause undesired variations in stresses exerted on the blocks by the reticulate grid. By this invention, clip elements are utilized to compensate for differences in cross-section as well as cooperating with the reticulate member to stress the blocks therein.

In the drawings:

FIGURE 1 is a perspective of a segment of a reticulate grid with a release agent coated on the walls of each cell of the grid.

FIGURE 2 is a vertical cross-section of a portion of the grid of FIGURE 1 wherein concrete blocks are molded in the cells thereof.

FIGURE 3 is a perspective of three adjoining walls of the grid of FIGURE 1, without the release agent, showing a U-shaped clip element placed over one wall and completely covering both sides thereof.

FIGURE 4 is a vertical cross-section of the completed prestressed structural unit of this invention.

Referring now to FIGURE 1 of the drawings, the numeral 10 denotes a segment of a reticulate metal grid member having a plurality of hexagonal cells 12 each defined by walls 14. Each wall of each cell 12 is coated with a thin layer of a release agent 18 exaggerated in thickness for purposes of illustration. The release agent may be a silicone or any other material which will perform the function to be later described.

Each cell 12 is now filled with concrete 20 or any other substance whose compressive strength is greater than its tensile strength. As indicated at FIGURE 2 of the drawings, the upper periphery 22 of each concrete cell element 20 may be chamfered for a purpose to be later described. The concrete is now allowed to set and harden.

After each concrete cell element 20 has hardened, the reticulate grid element 10 is lifted off of the concrete cell elements leaving them in substantially the same position as they were when within the reticulate grid. The release agent 18 allows for easy sliding of the walls 14 from between the cells 20. Now, each cell 20 exactly conforms to its corresponding cell 12, any cross-sectional variations of the latter having been imparted to the former.

Each wall 14 of the grid element 10 is now provided with a U-shaped metal clip 24, as shown for one wall in FIGURE 3, which may be placed over either th top or the bottom edges of the cell walls 14 and lies parallel and covers completely the surfaces of these walls. The reticulate member 10, together with the clips 24, are now heated. Each cell 12 thus expands, increasing its crosssection. While at the elevated temperature, the grid member 10, together with the associated clips 24, are placed over the plurality of concrete cell elements 20 with the edges of the walls 14 aligned with the chamfered portions 22. The grid is now lowered so that the resultant grid and concrete elements are all in the same plane. Chamfered portions 22 facilitate lowering of the grid onto and thence between the blocks, the lower edges of the grid being guided to positions between the blocks upon abutting the chamfered portions upon initial contact with the blocks upon lowering. The grid 10 and clips 24 are now allowed to cool and upon cooling the cross-section of each cell 12 diminishes thus circumferentially compressing each concrete element 20.

The function of the clips 24 is as follows. Being of only slight thickness (in the order of four or five mils), the cross-section of the cells 12 when heated and with the clips in place is only slightly greater than that of the molded concrete cells or hex blocks 20. Thus, after cooling, the cross-section of the cells 12 with the clips is smaller than that of blocks 20 and the latter undergo circumferential stress. Because the thickness of the clips is slight, any irregularities in them are squeezed out by the compressive force of the grid walls 14 on the peripheries of the blocks 20.

An alternative method of fabricating the prestressed structural unit of this invention is as follows. The release agent 18 may be dispensed with and each cell 12 is filled with concrete 20 or other substance whose compressive strength is greater than its tensile strength. Again, the upper periphery 22 of each concrete cell element 26 may be chamfered if desired. The concret is allowed to set and harden. The metal reticulate grid is now heated rapidly, as by the passage of electric current therethrough, and each cell 12 in the grid member undergoes crosssectional expansion. Because the heating is relatively rapid, the metal expands more rapidly than the concrete blocks in the individual cells and after a period of time there exists a clearance between each wall of each concrete block and the walls of the cell in which it is located. If desired, the heating may be slower, as by heating in the interior of an oven. The metal, having the greater coefficient of expansion, will expand more than the concrete.

At this point in the fabrication, the grid being expanded, the shims 24 are now placed straddling each wall 14 and each leg of each shim thereby occupying the clearance space or opening between the walls of the concrete and the walls of the reticulate grid. The electric current is now discontinued and, upon cessation of the current, the reticulate grid shrinks and the concrete blocks 20 are compressed around their peripheries.

Chamfered portions 22 facilitate the insertion of shims 24, but are not essential.

Since, in general, the walls of the shims 24 will not contain the same departures from a true plane as the grid walls 14 and the matching peripheral walls of the blocks 20 (the departures inherent in the manufacture of the grid 10), it is preferable that the material from which the shims are fashioned be softer than both the grid walls and the blocks 20. In this manner, the shims will assume the precise form, upon compression, as the walls between which they lie. The compressive forces exerted by the grid walls are thereby undistorted by the shims. Tin shims have been found suitable.

I claim:

1. A method of fabricating a prestressed structural member including the steps of pouring concrete into a plurality of the cells in a recticulate member, a complete circumferential Zone of each cell contacting the concrete, allowing the concrete to harden, expanding each of the said cells in the reticulate member until there exist spaces between the walls of the hardened concrete blocks and the cells in which they are positioned, inserting shim elements into said spaces and then allowing the reticulate member to contract.

2. The method of claim 1 wherein the expansion of the reticulate member is effected by electrical resistance heat- 3. The method of claim 1 wherein the shim elements are of a material softer than both the material from which the reticulate member is formed and the concrete.

4. The method of claim 1 wherein the reticulate member is formed of metal.

5. The method of making a prestressed structural unit comprising the steps of cross-sectionally expanding the cells of a reticulate member, placing thin shims on the walls of the cells, placing blocks of the same cross-sectional configuration as that of the cells before the latters expansion in the cells, and causing the expanded cells to contract, whereby the blocks are compressed over a continuous peripheral zone.

6. The method of making a prestressed structural unit comprising the steps of placing concrete in its flowable state in a plurality of the cells of a reticulate member, allowing the concrete to harden to thereby form blocks, removing the reticulate member, cross-sectionally expanding the cells of the reticulate member and placing thin shims on the walls of the cells of the reticulate member, placing the molded blocks in their corresponding cells, and causing the expanded cells to contract, whereby the blocks are compressed over a continuous peripheral zone.

References Cited FOREIGN PATENTS 1,095,395 12/1954 France.

ROBERT F. WHITE, Primary Examiner.

J. H. SILBAUGH, Assistant Examiner. 

